The present invention relates to the selective, preferably anisotropic, etching of films such as silicon nitride, formed over silicon dioxide. In particular, our invention relates to a process for etching silicon nitride anisotropically with high selectivity to the underlying silicon dioxide and with a controlled silicon nitride profile and controlled silicon dioxide loss or gain.
The composite silicon nitride-over-silicon oxide has a number of applications in integrated circuits. For example, nitride-over-oxide may be used as an oxidation mask to selectively expose field areas of IC (integrated circuit) wafers during LOCOS formation of the field isolation oxide. In such applications, the underlying oxide may be very thin. During the patterning of the silicon nitride LOCOS mask, it is necessary to precisely replicate the lithographic mask, such a photoresist, in the silicon nitride without dimensional loss, with good resolution and without etching of (i.e., with suitably high selectivity for) the overlying photoresist mask and the underlying thin oxide. When the nitride is removed subsequently from the active areas, selectivity for photoresist is not a concern, because the photoresist is stripped prior to the LOCOS oxidation. However, preventing degradation or removal of the thin oxide is still very much a concern.
Fluorocarbon etchants such a CH.sub.3 F and CH.sub.2 F.sub.2 and SF.sub.6 and NF.sub.3 have been used to etch silicon nitride. Flurocarbon etchants may form non-volatile carbon-containing organic polymers which deposit on the oxide and, therefore, increase the etch selectivity. However, carbon forms particulates and, thus, results in an undesirably dirty process, particularly for LSI (large scale integration) and VLSI (very large scale integration) circuits, which are susceptible to even a relatively few very small particulates. In addition, organic polymer layers may deposit, sometimes indiscriminately, within the reactor, necessitating relatively frequent cleaning of the reactor.